20091119 Camber, the Johanna way.pdf - The Junk Rig Association

Camber, the Johanna way.A discussion of Hows and Whys around the barrel-cut cambered panel junk sailMalena Johanna Samson BroremannOver the years I’ve received a number of questions (expressing worries) about the camberedpanel sails I have designed for boats like Malena, Johanna, Samson and Broremann. All ofthese sails have got their cambered panels from the so-called barrel cut method.The paper template for Broremann’s lower panels. Note the rounding along battens; the barrel shape.Sail material.One misunderstanding which almost has grown into a dogma, is that the barrel cut methodonly will work with very elastic sailcloth. This is not so. The blue cloth used on Johanna isquite stretchy diagonally, but not to any abnormal degree. It is definitely not a sort of1(4)

navalised Lycra! Some years ago Don Taylor sent me a sample of the Odyssy III cloth and Ifound it to be just as stretchy as my blue cloth. In fact, when studying both cloths under amagnifying glass, it appeared to me that they were woven in the same way. In both cases Ifound them impossible to rip with hand force.The German yacht Peregrine has a big JR with barrel cut panels. AFAIK it has been madefrom ordinary Dacron sail cloth, not very light either. That boat sails very well.In Peregrine’s sail there are not many wrinkles along the battens. I think Sebastian has stretched the saila bit more along the battens than I do. This limits the camber a bit, but also makes the vertical curve moretrapeze-like than in my sails with slacker setup. We had a hard time overtaking Peregrine in Johanna...Vertical curve.One argument I often hear against the barrel cut method is that one only gets the designedcamber on a little spot mid-between the battens.Broremann’s barrel cut panelsKingfisher 26, boom-shelf cut panels2(4)

The diagram above shows the curves at the vertical cut through the max camber point in 3 different sails.When I made the first barrel-cut sail for Malena, I too was concerned that only a part of thesail area would actually be working. I already had experience with a flat sail and hingedbattens sail (same size), so was curious about how this one would compare. I needed not haveworried; the cambered panel sail was at least as fast to windward as the hinged batten sail.One may think that the reason for Malena’s and Johanna’s success is mainly the high SA/dispratio. Not so with the schooner Samson. Despite having a SA/disp of just 13.3 she has provento be a good sailer, even in fairly light winds (F3).I can’t prove it yet, but my hunch is that this is what is happening at the barrel-cut panels: Close to the luff, the Johanna-style batten panels indeed have a trapeze shape with avery narrow groove at the battens. The incoming wind will thus see an almost uniformleading edge. In other words, the angle between the incoming wind and leading edgeis almost constant. The same happens at the trailing edge: The vertical curve between the battens is nearlystraight (trapeze or rectangle). This forces the air to leave the sail with the same angleof departure. I’m not sure what happens in the middle (max camber position), but my hunch is thatthe viscosity of the air ensures that the air passing over the groove more or less takesthe same route as the air passing over the hump. By the way; there is no guaranteethat the wind will just pass along the batten panels – it may well pass diagonally forwhat I know.The key thing in my opinion is how the wind enters and leaves the sail. My telltales at theleech definitely indicate a healthy airflow across the sail (until I stall it by over-sheeting it),but I see now that I should also fit telltales at each batten, not only at the middle of each3(4)

panel, as today. If these extra telltales indicate a nice airflow, then I declare that the whole sailis working.In real life I don’t want more total force out of Johanna’s sail than I have today. With today’ssetup with 8% camber (or should I say “8” ?) and a SA/disp=23, I can carry almost as muchsail upwind as downwind. If I increased the camber, I would have to reef earlier when closehauledand then I would have to un-reef each time I fell off to a run. With my sort of sailing,in and out among islands, that would be quite awkward. I could however use a more closewindedrig, but am unwilling to pay for it in money or trouble (read: carbon mast or wingsail).Nothing negative on the boom shelf method: It may well give at least as powerful sails as thebarrel cut method and it may also end up looking better. But amateur-friendly the boom shelfmethod is not. Sewing straight edges to curved ones isn’t easy.ConclusionMost methods of making camber seem to work somehow. The important thing if one needbetter progress to windward is to say goodbye to the flat sails.Stavanger, 20091119Regards fromArne Kverneland(a-kve2@online.no).. winter is when the Fuji heater is back on board. That happened last week so now I am ready to fry flieson the walls again...4(4)